1. Field of the Invention
This invention relates to securing threaded fasteners in engagement within a steam turbine and more particularly to means for locking a nozzle block to a nozzle chamber.
2. Description of the Prior Art
It is good turbine engineering practice to use a locking device of some sort on all fasteners, such as screws and bolts that are within the turbine. Turbine unbalance vibration and steam flow excitation can loosen fasteners if locking devices for the fasteners are not utilized. The most commonly used steam turbine locking devices are pins and pant leg washers. Other locking devices that can be used for fasteners are threaded friction plugs, lock wires, cotter pins, and tack welding. In some areas which are not conducive to using the normal locking devices, turbine protection is sometimes obtained by trapping the fastener.
Turbine nozzle block cap screws are especially prone to loosening and, in some cases, come completely free causing damage to other tubine parts. In the past, the following have been used as locking or containment devices: retaining rings, end plugs, and caulking strips. None of these devices were completely successful in locking the nozzle block cap screws. The most recent locking method used was pinning each nozzle block cap screw. This provided a positive locking device but caused some problems in the assembly and removal of the pins.
During assembly, the nozzle block cap screws are inserted through the nozzle block and into threaded engagement with the nozzle chambers, or alternately from the opposite side, through the nozzle chamber and into threaded engagement with the nozzle block. The nozzle block and nozzle chamber have predrilled holes provided for the nozzle block cap screw pins. The pin holes in the cap screw heads cannot be drilled until the cap screws have been torqued. At that time, a pin hole must be hand drilled through the cap screw head in alignment with the predrilled hole in the nozzle block or nozzle chamber. The hand drilling is a difficult field operation because of the hardened cap screw material. Hand reaming of the drilled holes is also necessary to obtain the required pin fit because if the fit is too loose, no locking action is obtained and if the fit is too tight, the pin may hang up during assembly. After assembly, the pin projects through the nozzle block or nozzle chamber pin hole, through the hand drilled hole in the cap screw head, and into the exposed internal hex indentation in the cap screw head.
For cap screw removal, the pin length projecting into the internal hex of the cap screw head must be sheared off before a wrench can be inserted into the internal hex indentation. The remaining pin length extending from the nozzle block or nozzle chamber into the cap screw is sheared off by removing the cap screw from the nozzle block or nozzle chamber with the wrench. When cap screw removal has been accomplished, the pin length remaining in the nozzle block or nozzle chamber must be either driven into the cap screw's hole in the nozzle block and sheared off or drifted out the opposite direction. This procedure of driving and shearing must be repeated until the entire length of the pin has been removed from the nozzle block or nozzle chamber. Occasionally, the pin becomes lodged in the pin hole and can only be removed by drilling. This procedure is time-consuming if required, and a pin of larger diameter is then necessary for reassembly.
During reassembly of the previously-used and drilled nozzle block cap screw, it is highly unlikely that the predrilled pin holes in the nozzle block or nozzle chamber and in the cap screws will be in alignment after the nozzle block cap screws have been tightened to the required torque. Possible solutions for obtaining alignment are: increasing the depth of counterbore for the cap screw head, removing material from the cap screw head's seating face, or drilling a new hole in the cap screw head. These solutions are undesirable from reliability or time consumption considerations.